Read the license agreement and then select Accept terms of the license agreement, if appropriate.

Press Next.

Press Finish.

The ADT plugin is not signed; you can accept the installation anyway by pressing Install All.

Restart Eclipse.

After restart, update your Eclipse preferences to point to the SDK root directory ($SDK_ROOT):
Select Window > Preferences… to open the Preferences panel. (Mac OS X: Eclipse > Preferences)
Select Android from the left panel.
For the SDK Location in the main panel, press Browse... and find the SDK root directory.

Press Apply, then OK

Updating the ADT Plugin

To update the ADT plugin to the latest version, follow these steps:

Select Help > Software Updates > Find and Install….

Select Search for updates of the currently installed features and press Finish.